Formation of cigs absorber layer materials using atomic layer deposition and high throughput surface treatment

ABSTRACT

An absorber layer may be formed on a substrate using atomic layer deposition reactions. An absorber layer containing elements of groups IB, IIIA and VIB may be formed by placing a substrate in a treatment chamber and performing atomic layer deposition of a group IB element and/or one or more group IIIA elements from separate sources onto a substrate to form a film. A group VIA element is then incorporated into the film and annealed to form the absorber layer. The absorber layer may be greater than about 25 nm thick. The substrate may be coiled into one or more coils in such a way that adjacent turns of the coils do not touch one another. The coiled substrate may be placed in a treatment chamber where substantially an entire surface of the one or more coiled substrates may be treated by an atomic layer deposition process. One or more group IB elements and/or one or more group IIIA elements may be deposited onto the substrate in a stoichiometrically controlled ratio by atomic layer deposition using one or more self limiting reactions.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of commonly-assigned co-pending U.S.patent application Ser. No. 10/943,658, which was filed Feb. 19, 2004,the entire disclosure of which are incorporated herein by reference.This application is a continuation-in-part of commonly-assignedco-pending U.S. patent application Ser. No. 10/782,545, which was filedFeb. 19, 2004, the entire disclosures of which are incorporated hereinby reference. This application is also related to commonly-assigned,co-pending application Ser. No. 10/782,233, titled “ROLL-TO-ROLL ATOMICLAYER DEPOSITION METHOD AND SYSTEM”, which was filed Feb. 19, 2004, theentire disclosures of which are incorporated herein by reference. Thisapplication is also related to commonly-assigned co-pending applicationSer. No. 10/782,017, titled “SOLUTION-BASED FABRICATION OF PHOTOVOLTAICCELL”, which was filed Feb. 19, 2004, the entire disclosures of whichare incorporated herein by reference. This application is also relatedto commonly-assigned co-pending application Ser. No. 10/943,685, titled“FORMATION OF CIGS ABSORBER LAYERS ON FOIL SUBSTRATES” (Attorney DocketNSL-038), which is filed Sep. 18, 2004, the entire disclosures of whichare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed to the deposition and/or surfacetreatment of thin films on large area substrates and more specificallyto atomic layer deposition in a high-throughput production system.

BACKGROUND OF THE INVENTION

Low-cost production of solar cells on flexible substrates using printingor web coating technologies is promising highly cost-efficientalternative to traditional silicon-based solar cells.

A typical Copper-Indium-Gallium-diSelenide (CIGS) solar cell structureincludes a back electrode followed by a layer of molybdenum (Mo). A CIGSabsorber layer is sandwiched between the Mo layer and a CdS junctionpartner layer. A transparent conductive oxide (TCO) such as zinc oxide(ZnO_(x)) or tin oxide (SnO₂) formed on the CdS junction partner layeris typically used as a transparent electrode.

A central challenge in constructing a highly efficiency CIGS-based solarcell is that the elemental components of the CIGS layer must be presentin a fairly narrow stoichiometric ratio relative to one another in orderfor the resulting cell to be highly efficient. It is difficult tocontrol the stoichiometric ratios and achieve high volume productionwith prior art techniques. Furthermore, it would be highly desirable tovary the composition of a CIGS absorber layer as a function of depth.There are numerous advantages to varying the relative concentrations ofthe components of the CIGS absorber layer. These advantages include (1)improved open circuit voltage; (2) improved short circuit currentdensity; and (3) improved optoelectronic quality in the absorber layer.A detailed discussion of these and other advantages may be found in OlleLundberg in “Band Gap Profiling and High Speed Deposition of Cu(In,Ga)Se₂ for Thin Film Solar Cells”, Comprehensive Summaries of UppsalaDissertations From the Faculty of Science and Technology 903, ActaUniversitatis Upsaliensis, Uppsala, Sweden 2003, which is incorporatedherein by reference.

More specifically, higher amounts of Ga deposited at or near the backcontact (e.g. near the Mo interface) of the CIGS cell promotes improveddevice function in two ways: (1) smaller grains form in the presence ofGa in this back region, and these smaller grains are less-mechanicallystressed at the back contact, thus improving the mechanical stability ofthe cell and (2) the presence of higher levels of CuGa at the back ofthe absorber layer also acts as a carrier reflector, directing carriersforward to the junction at the front of the absorber layer.

Second, a relatively high level of Ga in the middle of the CIGS absorberlayer negatively impacts device function, as small CuGaSe grains form.These small grains tend to have a high defect density and act as sitesfor charge recombination in the absorber layer.

Third, high amounts of Ga deposited at or near the front contact (e.g.near the TCO layer) of the CIGS cell can promote improved devicefunction in two ways: (1) a higher bandgap (e.g. 1.35 eV) near the frontcontact sets the voltage of the cell at a relatively higher value thanwould otherwise exist, and (2) such a higher voltage couples with alower current results in the same power conversion efficiency but withfewer I²R losses.

In the prior art, graded bandgap devices with graded concentrationprofiles have been prepared using co-evaporation from elemental and/oralloy sources, and have produced the best performing CIGS solar cellsrecorded to date. For example, Ramanathan and coworkers at the NationalRenewable Energy Laboratory showed a cell having a 19.2% conversionefficiency with a fill factor of 78.12%, Jsc=35.71 mA/cm², and anopen-circuit voltage of 0.69 V using this approach (see K. Ramanathan etal., “Properties of 19.2% Efficiency ZnO/CdS/CuInGaSe₂ Thin-Film SolarCells”, Progress in Photovoltaics: Research and Applications. Vol. 112003, pp. 225-230).

Unfortunately, there are several challenges and disadvantages associatedwith evaporation as a deposition technique for the CIGS absorber layer.For example, it is difficult to deposit many compounds and alloycompositions by evaporation. Evaporation deposition is limited toline-of-sight and limited-area sources, which tends to result in poorsurface coverage and spatial non-uniformity. Line-of-sight trajectoriesand limited-area sources can also result in poor film-thicknessuniformity over large areas. In addition, evaporation depositiontechniques typically have relatively few processing parameters that canbe varied to tune the properties of the resulting film. Furthermore, theintermixing of the elements from evaporative sources occurs not onlyatomically but also as the interaction of discrete atomic clusters,resulting in locally non-uniform deposition in all three dimensions.Such non-uniformity also alters the local stoichiometric ratios of theabsorber layer, decreasing the potential power conversion efficiency ofthe complete device. Further, the use of evaporation can result inuneven surfaces that can further degrade the device performance.

Moreover, the use of evaporation as a deposition mechanism to grade thebandgap provides at best a rough tuning. It is extremely difficult, ifnot impossible to reproducibly control the gradation of the individualcomponents of the absorber layer at the atomic or sub-monolayer level.Such reproducible control would be desirable for the formation ofprecisely structured bandgap gradients in the absorber layer.Reproducible, atomic-level gradation of the relative amounts ofdeposited elements and/or alloys would allow a higher-yield optimizationof the completed device. Finally, the creation of a bandgap gradedabsorber layer using evaporative sources requires a relatively expensivereal-time monitoring system to assess the relative composition of theabsorber layer as it is being constructed.

Chemical vapor deposition (CVD) of copper indium di-sulfide (CIS) filmswas first reported in 1992, when a single-source precursor was used inmetal organic CVD to produce a photo-responsive light absorbing film,though not in a solar cell (see Nomura, R., Seki, Y. and H. Matsuda.1992. Preparation of CuInS₂ Thin Films by Single-source MOCVD Processusing Bu₂In(SPr)Cu(S₂CNPr₂). J. Mater. Chem. 2 (7): 765-766. Since theelements in the film derived from a single source, the stoichiometricratio of these elements in the resulting films could not be alteredrelative to one another, leading to the inability to tune and optimizethe optoelectronic and electronic properties of the deposited film.Further, no bandgap grading was possible. Finally, sulfur rather thanselenium was used in the films, which limits the potential for largegrain growth. In particular, large grains only form in sulfur-containingCIS films in Cu-rich regions. In such copper-rich regions, coppersulfide forms, and this compound needs to be removed e.g. with apotassium cyanide etch for proper device function.

More recently, Atomic Layer Deposition (ALD) has been used in thefabrication of a film comprised of aggregates of CIS-coated titaniaspheroids as the active layer of inorganic solar cells (see e.g., Nanu,M., Schoonman, J. and A. Goossens. 2004. Inorganic Nanocomposites of n-and p-type Semiconductors: A New Type of Three-Dimensional Solar Cell.Adv. Mater. 5 (5): 453-455 and Nanu, M., Reijnen, L., Meeester, B.,Schoonman, J. and A. Goossens. 2004. Chemical Vapor Deposition 10 (1):45-49) which are incorporated herein by reference. In this approach, theaggregated titania spheroids were coated with 25 nm of CIS film usingALD, a high-quality thin-film deposition technique based on sequential,self-limiting surface reactions. Atomic layer deposition works byexposing a substrate sequentially to two or more reactant vapors orsolutions and maintaining the substrate temperature within, for example,a temperature range that depends on the chemistry of the particular ALDreaction. A typical ALD process involves a sequence of two different andalternating surface reactions involving two different gaseous reactantsreferred to herein as A and B. The ALD system is typically purged ofreactant gas in between reactions with a non-reactive purge gas C,and/or is pumped clean of the reactant gases. Sequencing the reactionsprovides precision in the rate of deposition and allows the use ofhighly reactive reactants. With each reactant exposure, a self-limitingreaction occurs on the surface of the substrate if the substratetemperature is, for example, within the right temperature range, or ifalternative energy sources are provided, such as energetic ions ormolecules or atoms, ozone, plasma, UV light, etc.

ALD can control the thickness of deposited films at the level of anatomic or sub-atomic layer. Thus films deposited by ALD tend to beuniform over large areas. In addition ALD allows deposition of conformalfilms on structures having very high aspect ratios (e.g., >>10). A widevariety of materials may be deposited by ALD, including semiconductors,metals, oxides, nitrides, and other materials. ALD techniques can thusdeposit thin films one atomic layer at a time, in a “digital” fashion.Such “digital” build-up of material greatly simplifies thicknesscontrol, thus reducing both the complexity and cost of thin filmdeposition.

Many industries, such as the optoelectronics industry, can benefit fromthe high uniformity, high aspect ratio conformal coating abilities andlow cost of ALD. Unfortunately, prior art ALD systems have mostly beenmade for semiconductor wafer processing, which is oriented to batchprocessed wafer handling systems. Although existing ALD systems aresuitable for the semiconductor industry, they are unsuitable for highvolume manufacturing of large area devices such as photovoltaic cells.Current systems are typically designed to coat small area wafers.Scaling up systems that coat a small area at a time might not bepractical for coating large area sheets, panels or rolls of material.ALD may be too slow and expensive overall, if only small area batchprocessing can be performed. Further, surface treatments such asannealing, drying, and exposure to reactive gases cannot be carried outat high-volume for large-area substrates when the surface treatmentsand/or reactions take place in a relatively smaller treatment chamber.

In particular, current ALD deposition approaches for the construction ofthin-film absorber materials in CIS solar cells are limited torelatively thin films, e.g. less than 25 nm, since thicker deposits ofabsorber material are impractical to form due to the slow rate ofdeposition. Thin films of CIS material are not optimal as an absorbercomponent for solar cells, since the majority of the light reaching a 25nm thick absorber layer is not absorbed by that material, limiting thepotential power conversion efficiency of such a solar cell. Further, CISabsorbers have a more narrow bandgap and tighter processing conditionsthan CIGS absorbers, where Ga is incorporated into the crystal structureof the absorbing semiconductor layer. Finally, to achieve an efficientpower conversion efficiency, the relative ratios of the individualelements comprising a CIS or CIGS film within a solar cell require agraded distribution through the absorber layer depth. This is verydifficult or impossible to achieve with such a thin (e.g. 25 nm)absorber layer, for which it is inherently challenging to establishinitial elemental gradients with a proper distribution, and where atomicintermixing at that length scale will tend to minimize any elementalgradients.

Thus, there is a need in the art, for a high throughput method andsystem for fabricating thick films of elementally-graded and optimizedCIGS solar cells produced by ALD.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a 3-dimensional exploded view schematic diagram of a highthroughput atomic layer deposition system according to an embodiment ofthe present invention.

FIGS. 2A-2D are a sequence of schematic diagrams illustrating formationof a CIGS absorber layer according to an embodiment of the presentinvention.

FIG. 3 is a schematic diagram of a photovoltaic cell according to anembodiment of the present invention.

FIG. 4 is a graph illustrating concentration profiles for a CIGSabsorber layer.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Although the following detailed description contains many specificdetails for the purposes of illustration, anyone of ordinary skill inthe art will appreciate that many variations and alterations to thefollowing details are within the scope of the invention. Accordingly,the exemplary embodiments of the invention described below are set forthwithout any loss of generality to, and without imposing limitationsupon, the claimed invention.

According to embodiments of the present invention, an absorber layer maybe formed on a substrate using atomic layer deposition reactions. Anabsorber layer containing elements of groups IB, IIIA and VIB may beformed by placing a substrate in a treatment chamber and performingatomic layer deposition of a group IB element and/or one or more groupIIIA elements from separate sources onto a substrate to form a film. Agroup VIA element is then incorporated into the film and annealed toform the absorber layer. The absorber layer may be greater than about 25nm thick. According to embodiments of the present invention, theabsorber layer thickness between about 25 nm and about 5000 nm, morepreferably 25 nm to 3000 nm, even more preferably 100 nm to 2000 nm,still more preferably, 500 nm to 1000 nm and most preferably 1000 nm to2000 nm. In some embodiments, a group IB element and two different groupIIIA elements (e.g., indium and gallium) may be deposited usingdifferent sources. In such embodiments the absorber layer may be betweenabout 1 nm and about 5000 nm thick.

In atomic layer deposition systems, wherever the reactant gases A, Bcome in contact with substrate, good coating may be achieved.Consequently, ALD may be readily scaled to coating large substrate areasin each reaction step. Thus, the throughput of an ALD system may beincreased by massively scaling the substrate surface area processedduring each step (as opposed to scaling up the step speed for eachdeposition cycle and/or processing many substrates in parallel via, e.g.the number of ALD reaction chambers) This can be achieved by coiling aflexible substrate (e.g., metal or alloy foil, e.g. Al, metalizedplastic foil, otherwise coated foils, foils with pre-deposited/processedsurface structure and/or patterning, laminates, etc.) in an ALD chamberin such a way that adjacent ‘turns or windings’ of the foil on thecarrier roll, cassette, or carousel do not touch one another. Gapsbetween adjacent layers of the coiled substrate allow the reactant gasesto flow or diffuse into the gaps between adjacent turns and thus reachand be adsorbed or react on the substrate surface(s) to be coated. Eachstep of a sequence of one or more ALD reactions may then be applied to amassive surface area of the substrate. The basic concept behind theembodiments of present invention is an implementation in which each stepmay be applied to the entire surface area of the substrate, e.g., to anentire roll. The same approach, with appropriate modifications, can beused to scale up other surface treatment techniques such as substrateannealing, drying, anodization, electro-deposition,electro-polymerization, electro-polishing, cleaning, exposing tochemicals to treat the surface (e.g. selenization of a substrate usingH₂Se gas or Se vapor), solution treatments, treatments that requireelectric fields/current/voltage, etc. Consequently, an entire CIGSabsorber layer as well as a window layer and front and back contacts maybe formed in one chamber without having to remove the substrate betweensteps.

According to embodiments of the present invention, an absorber layer foran optoelectronic device (e.g., a photovoltaic cell) may be fabricatedby atomic layer deposition of an absorber layer material containingelements from groups IB, IIIA and VIA on a coiled substrate. The coilingof the substrate increases the surface area that can be treated at onetime, thereby increasing throughput and yield. In general one or moregroup IB elements and/or one or more group IIIA elements are depositedby atomic layer deposition in a stoichiometrically controlled ratiousing one or more self limiting ALD reactions. Two or more precursorgases of the group IB and group IIIA elements may be intermixed in a mixratio that translates into a deposition ratio of the group IB and IIIAelements on the substrate. Alternatively, the group IB and group IIIAelements may be deposited by an atomic layer deposition sequenceinvolving two or more self-limiting single species deposition reactionswith precursor gases of the group IB and group IIIA elements. In eithercase, the group VIA element may be deposited by performing a reactionthat incorporates an element of group VIA into the absorber layer. Inaddition, embodiments of the present invention include combinationdeposition sequences involving both single species deposition reactionsand mixed species deposition reactions.

One way to achieve high-volume production in ALD systems of the typedescribed herein is to perform an ALD reaction on an entire roll ofsubstrate material at one time. To do this, it would be useful to fitthe entire length of a roll into the ALD chamber so that the A and Bhalf-reactions can be performed without having to remove the substratefrom the chamber. In another alternative embodiment, an entire roll ofsubstrate material may be treated at one time by using a system of thetype depicted in FIG. 1. The system 100 includes a surface treatmentchamber 102 and a carousel 104 for coiling a flexible substrate 106 in away that allows gaps between adjacent turns of the coil. Gas sources108, 110 and 112 provide ALD reactants A, B and purge gas C to thechamber 102. One or more robots 114 place stackable spacers 116 on thecarousel 104. An exhaust system 111 removes gas or liquid from withinthe chamber 102. The gas pressure within the chamber 102 may be adjustedby appropriate control of the gas sources 108, 110 and 112 and theexhaust system 111.

By way of example, the chamber 102 may include an inner wall 103 and anouter wall 105. The substrate 106 may be wound outside the chamber 102on the carousel 104, which fits between the inner wall 103 and the outerwall 105. The carousel 104 may be loaded into the chamber 102 throughthe top (or side). A lid 107 seals the top of the chamber 102. By way ofexample, the chamber 102 may also include equipment for pre-treatment ofthe substrate 106 by plasma, UV-ozone, heat (e.g., infrared), corona orcombinations thereof. In addition, the chamber 102 may include equipmentfor performing one or more treatment and/or coating steps that areperformed prior or subsequent to atomic layer deposition that isperformed in the chamber 102, including but not limited to substratecleaning, annealing, drying, and/or exposure to reactive gas such as Sevapor or H₂Se. Such post-ALD steps may include passivation, or coatingthe ALD treated substrate 106 with, e.g., an organic or inorganicmaterial. In addition, the chamber 102 may be equipped with additionalgas low inlets, heaters (e.g., infrared heaters, light sources, orultraviolet radiation sources, sources for energetic particles such asplasma ions, ozone, etc.) or cooling mechanisms, such as fluid filledtubes or peltier effect (thermoelectric) elements. Furthermore, thechamber 102 may be part of a much larger coating line that may includeother equipment for performing pre-ALD and post-ALD treatment of thesubstrate 106. The wound substrate can also be surface-treated in theabsence of ALD processes, e.g. by carrying out substrate cleaning,and/or annealing, and/or drying, and/or exposure to reactive gas (suchas Se vapor or H₂Se) in the absence of ALD. Further, the wound substratecan also be surface-treated in the absence of ALD processes, e.g. bycarrying out anodization, electrodeposition, electroplating,electropolishing, and/or other reactions in the absence of ALD. Thecarousel 104 and/or surrounding chamber may include an optionalsubstrate temperature control element (e.g., heating and/or coolingelement) to maintain the temperature of the substrate 104 within aspecified range.

Gas sources 108, 110, supply reactant gases A and B for sequentialatomic layer deposition processes that occur in the chamber 102. Gassource 112 may supply an optional non-reactive purge gas C, e.g., aninert gas such as argon or argon. Alternatively, or in addition,reactant gases A and B (or better reactant gases A and B each mixed intoa carrier gas such as nitrogen (N₂) or argon) could be removed viapumping. The gas sources 110, 112, 116 may selectively supply eitherreactant gas A or reactant gas B and/or purge gas C though one or moregas lines and one or more valves 113. Although three gas sources aredepicted in FIG. 1 for the sake of example, any number of gas sourcesmay be coupled to the chamber 102 as required to perform the desiredreaction or reactions. As described above, a temperature control elementmay be disposed in the chamber 102 or on the carousel 104 to control thetemperature of the substrate 106 and/or chamber 102. Alternatively or inaddition, other energy sources could be used, such as energeticparticles (from plasma, Ozone, etc.), UV light, etc. At the right rangeof temperature and/or presence of other energetic species and pressureeach reactant gas A, B may participate in a half-reaction at the surfaceof the substrate 106. When the two half-reactions are performedsequentially a very thin layer of material, e.g., as little as oneatomic layer or (more common) part of one monolayer, may be deposited onthe substrate 106 as a result of the two half-reactions.

A typical ALD process involves a sequence of two different andalternating surface reactions involving two different gaseous reactants.The first reaction exposes the substrate to a pulse of a precursor gascontaining molecules or atoms of interest that are to be deposited. Uponapplication of a pulse of precursor gas, the entire surface within theALD chamber becomes saturated with chemisorbed molecules of theprecursor gas. The atoms of interest attach the precursor gas moleculesto deposition sites on the substrate surface. The second reactionexposes the substrate and attached precursor gas molecules to a pulse ofsecond gas, typically a reducing agent, such as hydrogen, which reactswith the attached precursor gas molecules and removes undesiredcomponents of the precursor gas leaving the atoms of interest attachedto the surface at the deposition sites.

The ALD system is typically purged of reactant gases in between thesereactions with a non-reactive purge gas, such as argon or nitrogen,which serve to remove excess chemical species from the reaction chamber.The separate and pulsed application of the second precursor gas followedby the purge with non-reactive gas ensures that no gas-phase reactionstake place in the gas-phase. Rather, chemical reactions occur on exposedsurfaces within the ALD reaction chamber. The preceding sequence may berepeated with the original precursor gas or with a different precursorgas. Such a technique may readily be applied to formation of CIGSabsorber layers.

ALD thus permits a IB-IIIA-VIA absorber layer (e.g., a CIGS layer) to bebuilt up layer-by-layer, using stepwise deposition of partial atomicmonolayers during each application cycle, with the aggregate growth ratedirectly proportional to the number of reaction cycles rather than thepressure or concentration of precursor gases in the chamber. ALDtechniques can thus deposit thin films one atomic layer at a time, in a“digital” fashion. Such “digital” build-up of material greatlysimplifies thickness control, thus reducing both the complexity and costof thin film deposition. As a result, ALD provides a means for theuniform deposition of large surface areas within the ALD chamber withsubstantial control over film thickness, film uniformity, and highconformality, even for substrates with features exhibiting very highaspect ratios (e.g. 100:1). Importantly, while the ALD process istypically carried out at a deposition rate of less than 1 nm perdeposition cycle, and is thus a relatively slow process compared toother CVD deposition techniques, the ALD process can nevertheless bescaled to ultra-high surface areas within a reaction chamber by usingsuitably wound and coiled substrates, using techniques such as thosedescribed herein. As ALD provides for a uniform and conformal coatingeven over high surface areas, the use of ALD eliminates the inconstantevaporation rates commonly experienced during co-evaporation,sputtering, or CVD processing of metals and metal organic precursors.

Some high-volume batch processes, e.g., chemical bath deposition (CBD)and atomic layer deposition (ALD), could potentially coat or otherwisetreat both sides of the coiled substrate 106 at one time. However, itmay be desirable to coat only one side of the substrate 106. Coating orotherwise treating both sides can result in waste of valuable reactantsor may lead to extra processing steps such as removing unwantedcoatings. To avoid such waste or undesired processing, two substratesmay be attached together “back-to-back” to form a dual substrate having,in effect, two front sides with the back sides protected againstundesired treatment. Preferably, the substrates are attached in a mannerthat allows them to be separated from each other after processing. Byway of example the substrates may be attached with a low-strengthadhesive or electrostatic film applied to the back side of one or bothsubstrates. Alternatively, an edge where the two substrates join may besealed, e.g., with a tape, so that reactants cannot reach the back sidesduring processing. The dual substrate may then be wound into a coil andcoated such that both front surfaces are treated while the back surfacesare not. Processing the substrate in this fashion may reduce the wasteof reactants and may increase the area of the substrate that can beprocessed at one time.

As set forth above, coiled-substrate ALD techniques, such as thosedescribed above, may be used to deposit an absorber layer material foran optoelectronic device such as a solar cell. By way of example,absorber layer material may include Cu with In or Ga and Se or S in astoichiometric ratio of approximately CuIn_(1-x)Ga_(x)(S, Se)₂, where xis between 0 and 1. It should also be understood that group IB, IIIA,and VIA elements other than Cu, In, Ga, Se, and S may be included in thedescription of the IB-IIIA-VIA alloys described herein, and that the useof a hyphen (“—” e.g., in Cu—Se or Cu—In—Se) does not indicate acompound, but rather indicates a coexisting mixture of the elementsjoined by the hyphen. Where several elements can be combined with orsubstituted for each other, such as In and Ga, or Se, and S, inembodiments of the present invention, it is not uncommon in this art toinclude in a set of parentheses those elements that can be combined orinterchanged, such as (m, Ga) or (Se, S). The descriptions in thisspecification sometimes use this convenience. Finally, also forconvenience, the elements are discussed with their commonly acceptedchemical symbols. Group IB elements suitable for use in the method ofthis invention include copper (Cu), silver (Ag), and gold (Au).Preferably the group IB element is copper (Cu). Group IIIA elementssuitable for use in the method of this invention include gallium (Ga),indium (In), aluminum (Al), and thallium (Tl). Preferably the group IIIAelement is gallium (Ga) and/or aluminum (Al) and/or indium (In). GroupVIA elements of interest include selenium (Se), sulfur (S), andtellurium (Te), and preferably the group VIA element is either Se or S.

The proper choice of precursor materials is important for the ALDprocess to proceed effectively. Appropriate materials typically exhibitthe following features: (I) sufficient volatility at the reactiontemperatures, thermal stability with minimal or no self-decomposition,significant reactivity with the second precursor (reducing agent), andsubstantial insolubility of both precursors in both the product film andthe underlying substrate. Limited solubility can however be tolerated ifthe out-diffusion of a precursor material is rapid enough to go tocompletion during a short purging period. Limited thermal stability canalso be tolerated if the temperature ranges for the deposition processesare well controlled.

For the ALD-based synthesis of Mo layers, any of a variety of precursormaterials can be used, including but not limited to molybdenum chloride,molybdenum iodide, or other halides, molybdenum ethoxide, molybdenum VIoxide bis(2,4-pentandedionate), molybdenum hexacarbonyl, molybdenumdisilicide, and other organomolybdenum or organometallic precursors (forexample containing Si or Ge), and combinations or mixtures of the above.

For the ALD-based synthesis of tungsten layers, any of a variety ofprecursor materials can be used, including but not limited to tungstenchloride or other halides, tungsten ethoxide, tungsten silicide, andother organotungsten or organometallic precursors (for examplecontaining Si or Ge), and combinations or mixtures of the above.

For the ALD-based synthesis of vanadium layers, any of a variety ofprecursor materials can be used, including but not limited to vanadiumchloride, vanadium iodide or other halides, vanadium tri-n-propoxideoxide, vanadium triisopropoxide oxide, vanadium trisisobutoxide,vanadium III 2,4-pentanedionate, vanadium IV oxidebis(2,4-pentanedionate), vanadium IV oxidebis(heacafluoropentanedionate), vanadium IV oxide bis(benzoylacetonate),and other organovanadium or organometallic precursors (for examplecontaining Si or Ge), and combinations or mixtures of the above.

For the ALD-based synthesis of silicon dioxide layers, a wide variety oforganometallic precursors are available, including but not limited tohexafluorosilicates, metasilicates, orthosilicates, and otherorganosilicon or organometallic precursors (for example containing Ge),and combinations or mixtures of the above.

For the ALD-based synthesis of chromium layers, any of a variety ofprecursor materials can be used, including but not limited to chromiumchloride, chromium iodide, or other halides, chromium IIIbenzoylacetonate, chromium (III) heaxafluoropentanedionate, chromium IIIisopropoxide, chromium III 2,4-pentanedionate, chromium III2,2,6,6-tetramethylheptanedionate, chromium III trifluoropentanedionate,chromium II acetate, chromium III acetate, chromium III2-ethylheaxonate, and other organochromium or organometallic precursors(for example containing Si or Ge), and combinations or mixtures of theabove.

For the ALD-based synthesis of CIGS absorber layers, there are severalpossible precursors for each of the elements in the absorber layer. Forcopper, suitable precursors include but are not limited to Cu(I) andCu(II) compounds such as CuCl, copper iodide, or other copper halides,copper diketonates (e.g. Cu(I)-2,2,6,6,-tetramethyl-3,5,-heptanedionate(Cu(thd)₂)), Cu (II) 2,4-pentanedionate, Cu(II)hexafluoroacetylacetonate (Cu(hfac)₂), Cu(II) acetylacetonate(Cu(acac)₂), Cu(II) dimethylaminoethoxide, copper ketoesters, otherorganocopper or organometallic precursors (for example containing Si orGe)), and combinations or mixtures of the above. For indium, suitableprecursors include but are not limited to indium chloride, indiumiodide, or other indium halides, dimethylindium chloride,trimethylindium, indium 2,4-pentanedionate (indium acetylacetonate),indium hexafluoropentanedionate, indium methoxyethoxide, indiummethyl(trimethylacetyl)acetate, indium trifluoropentanedionate, andother organoindium or organomettalic precursors (for example containingSi or Ge), and combinations or mixtures of the above. For gallium,suitable precursors include but are not limited to diethylgalliumchloride, gallium triiodide, or other gallium halides, Ga (III)2,4-pentanedionate, Ga (III) ethoxide, Ga(III)2,2,6,6,-tetramethylheptanedionate, tris(dimethylaminogallium), andother organogallium or organometallic precursors (for example containingSi or Ge), and combinations or mixtures of the above. For aluminum,suitable precursors include but are not limited to aluminum chloride,aluminum iodide, or other halides, dimethylaluminum chloride, aluminumbutoxides, aluminum di-s-butoxide ethylacetoacetate, aluminumdiisopropoxide ethylacetoacetate, aluminum ethoxide, aluminumisopropoxide, aluminum hexafluoropentanedionate, Al(III)2,4,-pentanedionate, Al(III) 2,2,6,6-tetramethyl-3,5-heptanedionate,aluminum trifluoroacetate, trisisobutylaluminum, aluminum silicate, andother organoindium or organometallic precursors (for example containingSi or Ge), and combinations or mixtures of the above.

ALD-based synthesis of CIGS absorber layers may also (optionally) use ametal organic precursor containing selenium such as dimethyl selenide,dimethyl diselenide, or diethyl diselenide or a sulfur-containing metalorganic precursor, or H₂Se or H₂S, or other selenium- orsulfur-containing compounds, and combinations or mixtures of the above.

Furthermore, it may be advantageous to increase the concentration of Gain the CIGS film, for example at the front of a CIGS absorber layer, bydepositing a layer of Ga onto a just-deposited CI, CIG, or CIGS film,where the CI, CIG, or CIGS film was deposited using any of a variety oftechniques, including but not limited to solution-based printing,sputtering, evaporation, and the like. To do so, ALD can be carried outusing any of a range of suitable Ga precursors, including but are notlimited to diethylgallium chloride, gallium triiodide, or other galliumhalides, Ga (III) 2,4-pentanedionate, Ga (III) ethoxide, Ga(III)2,2,6,6,-tetramethylheptanedionate, tris(dimethylaminogallium), andother organogallium or organometallic precursors (for example containingSi or Ge), and combinations or mixtures of the above.

For the ALD-based synthesis of cadmium sulfide, any of a variety ofprecursor materials can be used, including but not limited to cadmiumchloride, cadmium iodide, or other halides, cadmium 2,4-pentanedionate,cadmium acetate, cadmium formate, dimethylcadmium, and otherorganocadmium or organometallic precursors (for example containing Si orGe), and combinations or mixtures of the above.

Other examples of specific A and B precursors and substrate temperatureranges for both in ALD of cadmium sulfide (CdS) include the followingshown in Table I:

TABLE I Substrate Temperature Range Precursor A Precursor B (° C.)Elemental Cd Elemental S 350-450 (source temp 320° C.) (source temp90-120° C.) CdCl₂ H₂S (5 sccm) 480 (source temp 470° C.)

Such CdS ALD is described e.g., by A. Kytokivi, A. et al., in MRS Symp.Proc. 222: 269-273, 1991, which is incorporated herein by reference.

For the ALD-based synthesis of Zinc oxide, any of a variety of precursormaterials can be used, including but not limited to zinc chloride, zinciodide, or other halides, zinc N,N-dimethylaminoethoxide, zincmethoxyethoxide, zinc 2,4-pentanedionate, zinc2,2,6,6-tetramethyl-3,5-heptanedionate, zinc acetate, zincbis(hexamethyldisilazide), and other organozinc or organometallicprecursors (for example containing Si or Ge), and combinations ormixtures of the above.

Other examples of specific A and B precursors and substrate temperatureranges for both in ALD of Zinc Oxide (ZnO) include the following shownin Table II below:

TABLE II Substrate Temperature Range Precursor A Precursor B (° C.)Dimethyl Zinc Trimethyl Ammonium 120-350 Diethyl Zinc Trimethyl Ammonium120-350

Such ZnO ALD is described e.g., by V. Lujala, in “Atomic layer epitaxygrowth of doped zinc oxide films from organometals” Applied SurfaceScience 82/83: pp 34-40, 1994, which is incorporated herein byreference. To react any of the above precursor materials on thesubstrate surface, ALD reactions require an additional reactant, often areducing agent or proton-donor compound. This compound can be introducedconcurrently with the first (precursor) reactant (especially if thecompounds do not cross-react prior to interacting with one another atthe substrate surface), or the introduction of the second reactant canbe made subsequent to the introduction of the initial (precursormaterial). When an organometallic precursor is hydrated, a proton-donorcompound may not be necessary. Reducing/proton-donating compoundsinclude but are not limited to water (H₂O), methanol, ethanol, isopropylalcohol, butyl alcohols, and other alcohols, and combinations ormixtures of these materials, as well as carbon monoxide (CO).

Oxygen gas (O₂) is also typically used as a second reactant, as is amixture of H₂O and H₂O₂.

For certain precursors, especially hexafluoro-pentanedionate (HFPD)precursors such as copper (II) hexafluoro-pentanedionate, indiumhexafluoro-pentanedionate, and gallium hexafluoro-pentanedionate,formalin (37% formaldehyde, and 15% methanol in distilled deionizedwater) is often used as the reducing agent while nitrogen gas (N₂) isused as the purge gas.

For these surface reactions, an inert gas (such as nitrogen, argon,helium) is typically used as the purge gas, while hydrogen gas is oftenused as a reducing agent.

In some situations, a seed layer, e.g., of platinum or palladium may bedeposited on the substrate before ALD with these precursors.

During the deposition process, a typical ALD cycle consists of 1-2seconds of a first metal organic precursor pulse, followed by a 1-2second purge, 1-2 seconds of a second metal organic precursor pulse,followed by a 1-2 second purge, (optionally) a 1-2 second pulse of athird metal organic precursor, followed by a 1-2 second purge, and(optionally) a 1-2 second pulse of a fourth metal organic precursor,followed by a 1-2 second purge, then (optionally) a 1-2 second pulse ofa fifth metal organic precursor, and finally (optionally) a 1-2 secondpulse of a sixth metal organic precursor. More generally, the durationof the pulse and/or purge cycles range from 0.001 seconds to 60 seconds,more preferably from 0.01 to 20 seconds, and most preferably from 0.1 to10 seconds.

The temperature used during ALD typically ranges from 150° C. to 600° C.depending upon the chemistry and physical properties of each precursormaterial.

Selenium and/or sulfur may be incorporated into the absorber layer inany of several ways. In one approach, ALD may be carried out within eachmonolayer of a nascent absorber layer deposit using precursor gases thatmay include a metal organic precursor containing selenium such asdimethyl selenide, dimethyl diselenide, or diethyl diselenide and/or asulfur-containing metal organic precursor, or H₂Se or H₂S. In this case,selenium is incorporated on a monolayer-by-monolayer basis as theabsorber film is built up through sequential deposition steps. Inanother approach, ALD of selenium-containing compounds can be carriedout on a periodic basis where the selenium precursor depositionfrequency is less than that required for initial incorporation into eachmonolayer. In yet another alternative approach, a nascent absorber layercan be exposed to selenium using either H₂Se, H₂S or selenium vapor. Inthis case, selenization can be carried out either (a) on a monolayer bymonolayer basis, or (b) periodically, where the exposure period islonger than a monolayer deposition cycles, or (c) at the end of theabsorber layer deposition sequence. Combinations and/or variations ofthese steps can be carried out. To carry out selenization and/orsulfurization, the film, if deposited on a flexible substrate, can bewound into a coil and the coil can be coated so that the entire roll isexposed at the same time, substantially increasing the scalability ofthe Se vapor exposure process through such a high-volume batch process,e.g., as described above.

An additional absorber layer reduction reaction may also optionally beperformed after each metal organic precursor pulse or after a sequenceof two or more pulses of precursor gas. Each of these approaches has itsadvantages and drawbacks. For example, when each precursor pulse isfollowed by a reduction reaction, the reduction process is likely to bemore effective since reactions take place on a relatively high surfacearea to volume ratio. However, the addition of repeated reduction stepscan slow the overall process speed. On the other hand, if reduction iscarried out only after a series of precursor pulses, the reductionprocess will likely be less effective since reactions take place on arelatively lower surface area to volume ratio, but eliminating therepeated reduction steps may increase the overall process speed. Thussome combination of these two approaches might work better for a givensituation. Alternatively, reduction can be carried out upon completionof the deposition of the other elemental components of the absorberlayer. A further complication arises from the need to ensure that thematerials that form during the deposition process are not locked into astochiometry and/or phase that cannot be later transformed into theintended target phase and stochiometry. Thus the deposition processshould be monitored and/or guided by the phase transformations that mayoccur as different combinations of elements are deposited within thenascent absorber film.

Embodiments of the invention are especially advantageous for theconstruction of solar cells based on CIGS absorber layers. For example,by use of ALD, copper, indium, gallium, and selenium can be deposited ina precise stoichiometric ratio that is intermixed at or near the atomiclevel. Furthermore, by changing sequence of exposure pulses for eachprecursor material, the relative composition of C, I, G and S withineach atomic layer can be systematically varied as a function ofdeposition cycle and thus depth with the product deposit. Both of thesefeatures can provide benefits (such as improved power conversionefficiency) as the product deposited serves within the absorber layer ofa solar cell. These benefits are much more difficult to achieve withconventional co-evaporation, sputtering, or solution-based deposition ofinorganic materials used for the absorber layers of solar cells, as boththe spatial uniformity and the potential resolution of atomically-gradeddeposition profiles of these processes is considerably less than forALD-based deposition. Further, the reproducibility of non-ALD-basedprocesses is less than that of ALD-based deposition.

Atomic layer deposition of a CIGS absorber layer material may proceed bythe sequence illustrated in FIGS. 2A-2D. Specifically, as shown in FIG.2A, a substrate 202 may have deposition sites 204 where atoms mayattach. After exposure to a copper precursor and reducing agent, copperatoms C occupy a portion of the sites 204 as shown in FIG. 2B. Othersites remain unoccupied. After exposure to an indium precursor andreducing agent, indium atoms I occupy a portion of the unoccupied sites204 leaving other sites unoccupied as shown in FIG. 2C. After exposureto a gallium precursor and reducing agent, gallium atoms G occupy aportion of the unoccupied sites 204 as shown in FIG. 2D. In thesedepositions, the amount of material deposited may be controlled, e.g.,by varying the number and sequence of relative exposure pulses of oneprecursor gas to another precursor gas, e.g. for the copper, indiumand/or gallium precursor. With ALD it is possible to precisely controlthe stoichiometric ratio in the CIGS absorber layer in several differentways. Specifically, the sequence of atomic layer depositions of the CIGScomponents can be varied in a way that controls the relativestoichiometric ratios of the elemental components of the deposited film.For example, a copper:indium stoichiometric ratio of 4:3 may be achievedby a deposition sequence of four copper depositions and three indiumdepositions, such as C-I-C-I-C-I-C or C-C-C-C-I-I-I or C-C-I-I-I-C-C andthe like, where “C” represents copper deposition and “I” representsIndium deposition. The sequence may be repeated as often as necessary toget the desired thickness. Here it is assumed that the copper and indiumdepositions deposit approximately equal numbers of atoms. Those of skillin the art will be able to devise other sequences that take into accountvariations in the deposition ratios. Furthermore, deposition sequencesinvolving more than two different CIGS components may be sequentiallyused, e.g. serial deposition of cycles of C-I-G or C-I-G-S, or C-I-A-S,or C-I-G-Se-Su, and so forth. In addition, deposition sequencesinvolving more than two different CIGS components may be performed inparallel, e.g. two or more metal-organic precursors can be introducedsimultaneously in the same deposition pulse. In a preferred mode,parallel deposition of more than one metal-organic precursor occurswithout reaction of those precursors prior to the surfacereaction/deposition.

In other embodiments of the invention, ALD may be used to control thestoichiometric ratio of a CIGS absorber layer as a function of depth.For example, if a 4:3 copper:indium ratio is desired over a firstdesired thickness at the bottom of the CIGS absorber layer and a 3:4copper:indium ratio is desired over a second desired thickness higher upin the CIGS absorber layer, then the C-I-C-I-C-I-C sequence may berepeated until the first desired thickness is achieved and a sequenceI-C-I-C-I-C-I may be repeated until the second desired thickness isachieved. Those of skill in the art will be able to devise sequencesthat take into account variations in the deposition ratios or that usemore than two different CIGS components. Stoichiometric ratios may alsobe varied as a function of depth by using a deposition sequence in whichthe particular pulse sequence for different precursor gases vary.

Once the various CIGS components (or more generally IB-IIIA-VIAcomponents) have been deposited by ALD they are usually annealed to forman absorber layer for a device. The annealing may be implemented byflash heating, also called rapid thermal processing. In particular, thesubstrate and absorber layer components may be flash heated at a rate ofbetween about 5 C.°/sec and about 5 C.°/sec to a plateau temperature ofbetween about 200° C. and about 550° C. The plateau time for the flashheating process may last between about 2 minutes and about 10 minutes.Such processing allows annealing of the absorber layer without damagingsubstrates that would otherwise be damaged by high temperature thermalprocessing. The combination of ALD and rapid thermal processing alsoallows for sharp transitions in the relative concentrations of theconstituents of the absorber layer. The rapid thermal processing istypically performed only once at the end of the absorber layerdeposition process. Selenization or sulfurization may be performedbefore or during annealing or both.

Although the preceding section describes deposition of copper, indiumand gallium by ALD, some of the components of the absorber layer may bedeposited by techniques other than ALD. It is noted here that suitablecontrol of the desired properties of the resulting absorber layer may beachieved even if only one component of the absorber layer is depositedby ALD. The other components may be deposited by conventional means.

A IB-IIIA-VIA alloy absorber layer formed as described above can be usedin an optoelectronic device 300, e.g., as shown in FIG. 3. The device300, which may be a solar cell, generally includes a substrate or baselayer 302, a base electrode 304, an absorber layer 306, a window layer308, and a transparent electrode 310. The base layer 302 may be madefrom a thin flexible material suitable for roll-to-roll processing Byway of example, the base layer may be made of a metal foil, such astitanium, a polymer such as such as polyimides (PI), polyamides,polyetheretherketone (PEEK), Polyethersulfone (PES), polyetherimide(PEI), polyethylene naphtalate (PEN), Polyester (PET), or a metallizedplastic. The base electrode 304 is made of an electrically conducivematerial. By way of example, the base electrode 304 may be a layer ofstainless steel, aluminum, or molybdenum, e.g., about 0.5 micron toabout 25 microns thick. An optional adhesion layer 303 may facilitatebonding of the electrode 304 to the substrate 302. By way of example,the adhesion layer 303 may be vanadium, chromium, tungsten or silicondioxide.

The absorber layer 306 may include material containing elements ofgroups IB, IIIA, and VIA. Preferably, the absorber layer 306 includescopper (Cu) as the group IB, Gallium (Ga) and/or Indium (In) and/orAluminum as group IIIA elements and Selenium (Se) and/or Sulfur (S) asgroup VIA elements. The absorber layer 306 may be fabricated using asequence of atomic layer depositions on the base electrode 304. Theabsorber layer 306 may be about 1000 nm thick. By using atomic layerdeposition as described above, the absorber layer 306 may be depositedat a temperature compatible with the underlying substrate 302 andelectrode 304. Furthermore, the elemental ratios of the IB, IIIA and VIAelements in the absorber layer 306 may be precisely controlled.

The window layer 308 is typically used as a junction partner for theabsorber layer 306. By way of example, the junction partner layer mayinclude cadmium sulfide (CdS), zinc sulfide (ZnS), or zinc selenide(ZnSe) or some combination of two or more of these. Layers of thesematerials may be deposited, e.g., by chemical bath deposition orchemical surface deposition, to a thickness of about 50 nm to about 100nm. The combination of the absorber layer 306 and the window layer 308is sometimes referred to as an absorber layer.

The transparent electrode 310 may include a transparent conductive layer309, e.g., a transparent conductive oxide (TCO) such as zinc oxide (ZnO)or aluminum doped zinc oxide (ZnO:Al), which can be deposited using anyof a variety of means including but not limited to sputtering,evaporation, CBD, electroplating, CVD, PVD, ALD, and the like. If thesubstrate is flexible and the deposition technique is ALD or CBD or thelike, a coiled/wound flexible substrate can be exposed so that theentire roll is processed at one time, e.g., as described above.

Alternatively, the transparent conductive layer 309 may include atransparent conductive polymeric layer, e.g. a transparent layer ofdoped PEDOT (Poly-3,4-Ethylenedioxythiophene), which can be depositedusing spin, sip, or spray coating, and the like. PSS:PEDOT is a doped,conducting polymer based on a heterocyclic thiophene ring bridged by andiether. A water dispersion of PEDOT doped with poly(styrenesulfonate)(PSS) is available from H. C. Starck of Newton, Mass. under the tradename of Baytron® P. Baytron® is a registered trademark of BayerAktiengesellscllatft (hereinafter Bayer) of Leverkusen, Germany. Inaddition to its conductive properties, PSS:PEDOT can be used as aplanarizing layer, which can improve device performance. A potentialdisadvantage in the use of PEDOT is the acidic character of typicalcoatings, which may serve as a source through which the PEDOT maychemically attack, react with, or otherwise degrade the other materialsin the solar cell. Removal of acidic components in PEDOT can be carriedout by anion exchange procedures. Non-acidic PEDOT can be purchasedcommercially. Alternatively, similar materials can be purchased from TDAmaterials of Wheat Ridge, Colorado, e.g. Oligotron™ and Aedotron™.

In addition to the transparent conductive layer 310, the transparentelectrode 310 may further include a layer of metal (e.g., Ni, Al or Ag)fingers 311 to reduce the overall sheet resistance.

For the optoelectronic devices of the type shown in FIG. 3, an optionalencapsulant layer (not shown) may provide environmental resistance,e.g., protection against exposure to water or air. The encapsulant mayalso absorb UV-light to protect the underlying layers. Examples ofsuitable encapsulant materials include one or more layers of polymerssuch as THZ (e.g. Dyneon's THV220 fluorinated terpolymer, afluorothermoplastic polymer of tetrafluoroethylene, hexafluoropropyleneand vinylidene fluoride), Tefzel® (DuPont), Tefdel, ethylene vinylacetate (EVA), thermoplastics, polyimides, polyamides, nanolaminatecomposites of plastics and glasses (e.g. barrier films such as thosedescribed in commonly-assigned, co-pending U.S. patent application Ser.No. 10/698,988, to Brian Sager and Martin Roscheisen, filed Oct. 31,2003, and entitled “INORGANIC/ORGANIC HYBRID NANOLAMINATE BARRIERFILM”), and combinations of the above.

Embodiments of the present invention encompass situations where theconcentrations of group IB, group IIIA and group VIA elements in theIB-IIIA-VIA absorber layer 306 vary with respect to depth. For thepurposes of the following discussion, the base electrode 304 and/oradhesion layer 303 is sometimes referred to as the “back” contact of thedevice 300. Similarly, the transparent electrode 310 is sometimesreferred to as the “front” contact of the device 300. It is also notedthat the base electrode 304 and adhesion layer 303 are sometimesreferred to as being at or near a “back end” of the device 300.Consequently, regions of the absorber layer 306 that are close to thebase electrode 304 and adhesion layer 303 are sometimes referred to as a“back region” of the absorber layer 306. Similarly, the window layer 308and transparent electrode 310 are sometimes referred to as being at the“front end” of the device 300 and regions of the absorber layer 306proximate the window layer 308 are sometimes referred to as a “frontregion.” A portion of the absorber layer 506 intermediate the front andback regions is referred to herein as a “central region”.

In one embodiment, the absorber layer 306 may include Cu, Ga, and/or Inand Se and/or S. As described above, the Ga content may be tuned to behigher at and near both the front and back contacts of the absorberlayer 306, but minimized in the central region of the absorber layer306, resulting in a “saddle” profile for Ga concentration as a functionof depth within the absorber layer 306, e.g., as shown in the graphdepicted in FIG. 4. In the example depicted in FIG. 4, an absorber layermay include a first Ga-rich region 402 in the vicinity of the back end,a second Ga-rich region 404 in the vicinity of the front end, arelatively Ga-deficient central region 406 between the first and secondGa-rich regions 402, 404 and a Se-rich region 408 in the vicinity of thefront end. By way of example, the first Ga-rich region 402 may becharacterized by a thickness of up to about 500 nm and a Ga to In+Garatio of between about 10% and about 90%. The second Ga-rich region 404may between about 5 nm and about 50 nm thick and have a ratio of Ga toIn+Ga of between about 20% and about 80%. The central region 406 has aratio of Ga to In+Ga of less than about 5%. The central region 406 maybe up to about 1000 nm thick.

The Indium concentration in the absorber layer may be characterized as aratio of Indium to Copper. In the first and second Ga-rich regions 402,404, the ratio of In to Cu may be roughly three parts In to one part Cu.The overall In level in the first Ga-rich region 402 and the Se-richregion 408 is typically less than in other regions of the absorberlayer. In the central region 406 the In:Cu ratio is roughly 0.88-0.92parts Cu to one part In. The concentration of Cu may be measured as anatomic ratio of Cu to the group IIIA elements (e.g., In and Ga). Thisratio may be as large as 90%. The ratio of Se to Cu may be roughlyconstant over most of the absorber layer but higher in the Se-richregion 408. For example over the first Ga-rich region 402, centralregion 406 and second Ga-rich region 404, the absorber layer may have acomposition of Cu_(0.9)Ga_(1-x)In_(x)Se₂. At the Se-rich layer 408, thecomposition may be CuIn_(3y)Ga_(3-3y)Se₅. In the specific case wherey=0, the Se-rich layer 408 may have a composition given by CuIn₃Se₅. TheSe-rich layer may be about 20 nm thick.

The thicknesses and concentration ratios set forth above are presentedfor the purpose of example and in no way limit the invention. Those ofskill in the art will recognize that the particular concentrations andthicknesses may be adjusted to optimize power conversion efficiency,open circuit voltage, short-circuit current density, fill factor, grainsize, charge mobility and other functional and/or structural parametersof the device 300.

Although in part of the preceding section the absorber layer 306 isdescribed as including copper, indium and gallium, those of skill in theart will recognize that the advantages of varying the stoichiometricconcentration as a function of depth may be extended more generally toabsorber layers of the IB-IIIA-VIA type. As such, embodiments of theinvention should not be limited to absorber layers containing copper,indium and gallium.

The advantages of the coiled substrate approach of the embodiments ofthe present invention may be illustrated by a numerical example.Consider a coiled substrate that can fit into 16′×16′×12′ space. Assumethat the coil has an inner diameter of 1 meter (e.g., for a hexagonalcarousel, the distance from the center to an edge of a hexagon). Assumethat the coil has an outer diameter of 3 meters and that the width ofthe coil is 2 meters and the carousel is a little wider, e.g., 2.4meters wide. Each turn of the coiled substrate is 1 meter long betweenthe edges of the hexagon initially and 3 meters long when fully wound.The average length of each turn of the coiled substrate is thus 6sides×2 m/side. If the substrate has a thickness of 0.025 mm andadjacent turns of the coiled substrate are 1 mm apart, then the coiledsubstrate would have about 2 m/1.025 mm=˜2,000 turns about the carousel.The total area of the coiled substrate would be 2,000×2 m×6×2 m=48,000m². In this numerical example, suppose that a deposition requires 25repetitions of the four step sequence ACBC, i.e., filling the chamberwith reactant A (e.g., a precursor), purge with inert gas C, fill withreactant B (e.g., a reducing agent), purge with inert gas C. If eachpurge, pump or fill step takes 10 minutes, the throughput may beestimated as the total area divided by the total number of steps and thetime per step, e.g., 48,000 m²/10 min/step/100 steps=48 m²/min>500square feet per minute. If the time for each step can be reduced to only1 minute, the throughput may be increased to >5,000 square feet perminute. Because of this relatively improved deposition rate, a thick(e.g. 20 nm to as much as 2000 nm or more) CIGS absorber layers can becost- and time-effectively deposited at high production volume. Thiscontrasts with the prior art, where, due to the limited deposition rateof flat substrates, ALD was used only for very thin films on extremelythin absorber (ETA) cells.

Furthermore, it is possible to form most or all of the layers of anoptoelectronic device by ALD in one chamber without having to remove thesubstrate from the chamber between steps. Specifically, with respect toa device of the type depicted in FIG. 3, the electrode layer 304 may bemade of molybdenum. By way of example, molybdenum may be deposited byALD, e.g., using MoCl₅ and Zinc (see e.g., M. Juppo, M. Vehkamaki, M.Ritala, and M. Leskela, Deposition of molybdenum thin films by analternate supply of MoCl ₅ and Zn, Journal of Vacuum Science &Technology A 16 (5), (1998) 2845, which is incorporated herein byreference). The absorber layer 306 may be deposited by ALD as describedabove, e.g., with respect to FIGS. 2A-2D, and then annealed by rapidthermal processing.

The window layer 308 may be Cadmium Sulfide (CdS) deposited by ALD.Cadmium Sulfide may be more reliably deposited by Chemical SurfaceDeposition, a technique described, e.g., in McCandless, B. E. and W. N.Shafarman. “Chemical Surface Deposition Of Ultra-Thin Cadmium SulfideFilms for High Performance and High Cadmium Utilization”, 3rd WorldConference on Photovoltaic Energy Conversion, Osaka, Japan, 2003, whichis incorporated herein by reference. CdS can also be deposited by aliquid-based atomic layer epitaxy, as described e.g. in T. E. Lister andJ. L. Stickney, “Formation of the first monolayer of CdSe on Au(111) byElectrochemical ALE”, Appl. Surface Science, 107 (1996), 153; and T. E.Lister, and J. L. Stickney, “CdSe Deposition on Au(111) byElectrochemical ALE,” Appl. Surface Sci., 103 (1996) 153.), thedisclosures of both of which are incorporated herein by reference. Evenwith such liquid based deposition techniques, the window layer 308 maybe deposited in the same chamber as the other layers if the depositionchamber is suitably configured to allow for filling and draining ofliquid phase reactants. Alternatively, the window layer 308 may be madeof materials other than CdS that may be deposited by ALD. Examples ofsuch materials include ZnO, Zn(O,S), ZnSe, In₂S₃, TiO₂, Ta₂O₅, andAl₂O₃, as described e.g., in the Sterner dissertation and otherreferences cited herein.

Finally, the transparent conductive layer 309 may be ZnO deposited byALD using diethyl zinc as a reactant and water vapor (H₂O) as a reducingagent. ZnO deposition by ALD is described e.g., by J. W. Elam et al,“Properties of ZnO/Al₂O₃ Alloy Films Grown Using Atomic Layer DepositionTechniques” Journal of The Electrochemical Society, vol. 150 no. 6, ppG339-G347 (2003), which is incorporated herein by reference.Alternatively, the transparent conductive layer 309 may be a conductivepolymeric layer, e.g., of the types described above, deposited from asolution, e.g., by any of a variety of coating methods including but notlimited to contact printing, top feed reverse printing, bottom feedreverse printing, nozzle feed reverse printing, gravure printing,microgravure printing, reverse microgravure printing, comma directprinting, roller coating, slot die printing, meiyerbar coating, lipdirect coating, dual lip direct coating, capillary coating, ink-jetprinting, jet deposition, spray deposition, and the like. Thetransparent electrode 310 may further include a layer of metal (e.g.,Ni, Al or Ag) fingers 311 to reduce the overall sheet resistance.

In embodiments of the present invention, the scaling of the ALD processis geared towards surface area maximization, not necessarily processstep speed, thus leaving enough time for each step. Consequently,pumping, purging and filling can be ensured to be high quality, thusminimizing loss of coating quality by intermixing gases, etc. Althoughthe whole process may take a considerable period of time to complete, avast surface area of substrate may be coated at one time.

While the above is a complete description of the preferred embodiment ofthe present invention, it is possible to use various alternatives,modifications and equivalents. Therefore, the scope of the presentinvention should be determined not with reference to the abovedescription but should, instead, be determined with reference to theappended claims, along with their full scope of equivalents. Theappended claims are not to be interpreted as includingmeans-plus-function limitations, unless such a limitation is explicitlyrecited in a given claim using the phrase “means for.”

1-10. (canceled)
 11. An in-line reactor to process a substrate accordingto a predetermined temperature profile, the reactor comprising; asubstrate inlet; a substrate outlet; a series of chambers between theinlet and the outlet, each chamber including: an upper body, a lowerbody, a gap formed between the upper body and the lower body, whereinthe gap includes a width, a height and a length, and wherein a ratio ofa narrowest width to a narrowest height for each chamber is at least 15,and wherein the gap of each of the series of chambers is aligned withthe gap of the other chambers in the series, and a temperaturecontroller that regulates the temperature within the gap based upon thepredetermined temperature profile so that there is a differenttemperature within the gap of at least some of the chambers; a mechanismto move the substrate from the inlet to the outlet through each gap ofthe series of chambers; and at least one gas inlet configured to delivera gas into the gap of a corresponding at least one of the chambers. 12.The reactor according to claim 11, wherein adjacent chambers areseparated by a buffer region.
 13. The reactor according to claim 12,wherein the gap height within at least one chamber varies across itswidth.
 14. The reactor according to claim 13, wherein the gap heightwithin at least one chamber varies across its length.
 15. The reactoraccording to claim 12, wherein the gap height within at least onechamber varies across its length.
 16. The reactor according to claim 11,wherein the gap height within at least some of the chambers isdifferent.
 17. The reactor according to claim 11, wherein the gap heightwithin each chamber is substantially the same.
 18. The reactor accordingto claim 11, wherein the temperature controller controls a heatingelement and a cooling element.
 19. The reactor according to claim 11wherein the mechanism includes a supply spool and a receiving spool thatare used to supply and receive, respectively, a flexible foil substrate.20. The reactor according to claim 12, further comprising a secondaryenclosure that contains the series of chambers and the mechanism. 21.The reactor according to claim 11 further including at least one ofSe-containing gas and S-containing gas connected to the gas inlet forsupplying at least one of Se and S to the gap.
 22. The reactor accordingto claim 11 wherein the gap height within the at least one chamber thatcontains the gas inlet is higher than an adjacent chamber that does notcontain any gas inlet.
 23. The reactor according to claim 11 whereineach of the series of chambers further includes a gap entrance, a gapexit, a gap entrance seal, a gap exit seal, and a second mechanism tomove the upper body and the lower body relative to each other between anopen position and a closed position, such that when in the open positionthe substrate is moved by the first mechanism, and when in the closedposition the gap is sealed by the gap entrance seal and the gap exitseal.
 24. The reactor according to claim 23, wherein at least one gasoutlet is associated with one of the chambers and is configured toremove a gas from the gap of the one chamber when the chamber is in theclosed position.
 25. The reactor according to claim 23, wherein adjacentchambers are separated by a buffer region.
 26. The reactor according toclaim 23, wherein the temperature controller controls a heating elementand a cooling element.
 27. The reactor according to claim 23 wherein themechanism includes a supply spool and a receiving spool that are used tosupply and receive, respectively, a flexible foil substrate.
 28. Thereactor according to claim 23, further comprising a secondary enclosurethat contains the series of chambers and the mechanism.
 29. The reactoraccording to claim 28 wherein the mechanism includes a supply spool anda receiving spool that are used to supply and receive, respectively, aflexible foil substrate.
 30. The reactor according to claim 23 furtherincluding at least one of Se-containing gas and S-containing gasconnected to the gas inlet for supplying at least one of Se and S to thegap. 31-50. (canceled)